1. Field of the Invention
The present invention relates to a liquid concentration sensing device that senses a concentration of a sensed liquid based on a capacitance.
2. Description of Related Art
In the case of an engine that uses ethanol-blended gasoline as fuel, generally, an ethanol concentration in the fuel is sensed with a liquid concentration sensing device provided in a fuel supply system, and fuel injection quantity, ignition timing and the like are controlled according to the ethanol concentration. Thus, deterioration of exhaust gas is inhibited and drivability is improved. The liquid concentration sensing device senses the ethanol concentration in the fuel by sensing a capacitance and fuel temperature between two electrodes immersed in the fuel. There has been a possibility that a conventional liquid concentration sensing device erroneously senses the ethanol concentration in the fuel when an abnormality occurs in a capacitance sensing device that senses the capacitance.
A measuring instrument described in Patent document 1 (PCT patent application Japanese translation No. H05-507561) senses an alcohol concentration by sensing a capacitance between two electrodes, which use a mixture gas consisting of alcohol and gasoline as a dielectric, in the form of an output frequency of an operational amplifier electrically connected to the electrodes. If the alcohol concentration lowers and the capacitance decreases, there is a possibility that the output frequency of the operational amplifier turns into an undetectable frequency. In order to prevent such the possibility, the measuring instrument has an additional capacitor connected with the two electrodes in parallel.
However, in the case where the measuring instrument is constructed as described in Patent document 1, the operational amplifier oscillates at a frequency corresponding to the additional capacitor if an abnormality such as a disconnection arises between the two electrodes and the operational amplifier. Accordingly, abnormality detection becomes difficult.